How GPS Knows Where You Are

Open a map app and tap “show my location.” Within a few seconds, a blue dot appears on the map, accurate to within a few metres of where you are actually standing. This works in the middle of a field with no nearby landmarks, in a city you have never visited, and in a moving car on a motorway. The technology responsible is GPS — the Global Positioning System — and it achieves this precision using nothing more than radio signals from satellites and some very precise arithmetic about time.

A Fleet of Satellites

GPS is operated by the United States Department of Defense, though it is freely available to any receiver on Earth. The system consists of at least 24 satellites arranged in six orbital planes, each satellite orbiting at an altitude of approximately 20,200 kilometres. The arrangement is designed so that at least four satellites are visible from any point on Earth’s surface at any time, under most conditions.

(GPS is one of several Global Navigation Satellite Systems; Russia operates GLONASS, the European Union operates Galileo, and China operates BeiDou. Modern smartphones can use signals from all of them simultaneously for better accuracy.)

Each satellite carries multiple atomic clocks — devices that track time by counting the vibrations of caesium or rubidium atoms, accurate to nanoseconds over years. These clocks are so precise that the satellites can be relied upon to broadcast timing signals that are accurate to roughly 20 to 30 nanoseconds.

The Key Insight: Distance from Time

GPS does not work by measuring signal strength or direction — it measures time. Each satellite continuously broadcasts a signal that contains two pieces of information: the satellite’s identity, and the precise time at which the signal was transmitted.

When your GPS receiver picks up that signal, it compares the signal’s embedded timestamp to its own clock. The difference between when the signal was sent and when it was received tells the receiver how long the signal spent travelling through space.

Radio waves travel at the speed of light: approximately 299,792 kilometres per second. If a signal took 0.067 seconds to arrive, the satellite must be 299,792 × 0.067 ≈ 20,087 kilometres away. This gives the receiver a distance measurement — not a direction, just a distance.

One distance measurement from one satellite tells you that you are somewhere on a sphere with a radius of 20,087 kilometres centred on that satellite. Not very useful on its own.

Trilateration: Narrowing It Down

With two satellites, the two spheres intersect in a circle. You are somewhere on that circle — still not precise enough.

With three satellites, the three spheres intersect at two points. One of those points is typically far out in space or below the Earth’s surface and can be discarded. That leaves one point — your position on the surface of the Earth. This is the principle of trilateration.

Trilateration is often confused with triangulation. Triangulation uses angles between known reference points to determine a position. Trilateration uses distances from known points. GPS uses trilateration.

In practice, three satellites give you a two-dimensional position (latitude and longitude). A fourth satellite is needed to determine altitude — your three-dimensional position — and to correct for errors in the receiver’s own clock (more on that shortly).

The Clock Problem

Atomic clocks are extraordinarily accurate, but they cost hundreds of thousands of dollars each. The GPS receiver in your phone does not contain one. It contains an ordinary quartz crystal oscillator, which can drift by microseconds per day.

A timing error of just one microsecond translates to a position error of about 300 metres, because the receiver would calculate that the signal had been travelling 300 metres further (or shorter) than it actually had. This would make GPS far too inaccurate to be useful.

The fourth satellite solves this problem. With position in three dimensions fully determined by three satellites, the receiver can work backwards from the fourth satellite’s signal to calculate exactly how much its own clock is drifting and correct for it. The fourth satellite essentially turns the receiver’s imprecise clock into a de facto atomic clock by synchronising it against the satellites’ known-accurate timing signals.

Atmospheric Interference

Even with four satellites and a corrected clock, errors remain. GPS signals must pass through two layers of the atmosphere on their way from orbit to your receiver.

The ionosphere — a layer of charged particles between 60 and 1,000 kilometres up — slows signals slightly in ways that vary with time of day, solar activity, and latitude. The troposphere closer to Earth’s surface also introduces small delays depending on temperature, humidity, and pressure.

These delays are not negligible: without correction, they would introduce errors of several metres. High-precision GPS receivers use dual-frequency signals (civilian receivers traditionally had access to one frequency; modern devices often have two or three), which allows the receiver to measure the difference in delay between frequencies and mathematically subtract most of the ionospheric error.

Assisted GPS

When you first open a navigation app after a period of the phone being off, you may notice a few seconds of delay before the blue dot appears. The receiver must first find satellites, download their ephemeris data (the orbital information that tells it where each satellite is in space), and collect enough signals to calculate a position. This initial acquisition can take up to a few minutes from a cold start.

Assisted GPS (A-GPS) solves this. Your phone downloads a compressed set of satellite orbital predictions from your mobile carrier’s network servers — a process that takes milliseconds over a data connection. Pre-armed with this information, the receiver knows exactly where to expect each satellite’s signal and can achieve a position fix in under five seconds.

Accuracy and Its Limits

Standard civilian GPS is accurate to roughly 3 to 5 metres under open-sky conditions. Several factors degrade this. Multipath errors occur in cities where signals reflect off buildings and arrive at the receiver having travelled a longer-than-direct path, tricking it about the satellite’s distance. Dense tree cover, tunnels, and indoor environments block signals altogether.

For applications that require centimetre-level accuracy — precision agriculture, land surveying, autonomous vehicles — a technique called Real-Time Kinematic (RTK) positioning uses a fixed ground station with a precisely known location to broadcast corrections to nearby moving receivers, removing atmospheric and clock errors almost entirely.

A Technology Built on Constraints

It is worth noting that GPS was designed and deployed during the Cold War for military navigation, and it shows. The system’s architecture — satellites, ground control stations, user receivers — is deliberately one-way. Satellites broadcast; receivers listen. Your GPS receiver does not send any signal back to the satellites, which means the system cannot track you (though your phone may share your position through the internet). It also means GPS works for an unlimited number of simultaneous users. Every receiver on Earth could be powered on at once and the system would not slow down in the slightest.